Allogeneic hematopoietic stem cell transplantation (HCT) is the only curative option for patients with certain hematologic malignancies, including acute myelogenous leukemia (AML), Despite precise matching between stem cell donor and recipient for HI_A genes, outcomes for more than half of patients remain disappointing due to post-transplant complications, such as leukemic relapse and viral reactivation. Activation of donor natural killer (NK) cells, key components ofthe innate immune system, can significantly prevent these complications and Improve HCT outcomes. NK response is dictated by interactions between the NK KIR receptors and their HLA ligands. Determining which KIR/HLA interactions most potently impact NK function and understanding how these Interactions take place is vital to the ultimate objective of selecting donors based on KIR and HLA to favor NK activation and improve HCT outcomes. The first aim ofthe proposal uses a unique transgenic mouse system and FRET technology to demonstrate that inhibitory KIR molecules interacting with HLA presented by adjacent cells (trans) as well as by the NK cell itself (cis) can both shape NK function. These results are directly relevant to predicting NK response in HLA-matched and mismatched HCT. The second aim is to show that the activating KIR2DS1 and KIR3DS1 contribute in an HLA-dependent manner to the functional capacity of the cell, explaining findings that KIR2DS1 in patients with the KIR ligand HLA-C1 is associated with lower relapse following HCT and that KIR3DS1 is associated with lower CMV reactivation. Findings from Alms 1 and 2 will support the concept that to capture NK activation In HCT, one must minimize NK inhibition. A novel donor selection algorithm based on KIR/HLA genotypes to maximize NK activity and improve HCT outcomes Is presented. The third aim seeks to apply this algorithm to donor selection, comparing outcomes post-selection with historic outcomes of HCT patients. Successful demonstration ofthe algorithm's feasibility and clinical impact will potentially change donor selection from an HIA-based strategy to a KIR/HLA-based strategy that will take advantage of innate immune effects against infection and relapse without increasing the risk of T-cell alloreactivity and graft-versus-host disease.